In 1950 Murphy U.S. Pat. No. 2,526,683 first described a process for preparing methyl cellulose medicinal capsules by a dip coating process using the apparatus described in U.S. Pat. No. 1,787,777 or similar dip coating apparatus. The process consists of dipping a capsule forming pin pre-heated to 40.degree.-85.degree. C into a cellulose ether solution kept at a temperature below the incipient gelation temperature (10.degree.-30.degree. C), withdrawing the pins at a predetermined withdrawal speed and then placing the pins in ovens kept at temperatures above the gelation temperature (45.degree.-85.degree. C), exposing the pins to a lower temperature first and then gradually to higher temperature until the film is dry. The dry capsule is then stripped, cut to size and the body and caps are fitted together.
The resulting methyl cellulose capsules had several advantages over conventional gelatin capsules including resistance to microorganisms and greater stability under extreme humidity conditions. However, these capsules failed to dissolve in the gastrointestinal fluid at body temperature in an acceptable time. Furthermore, the different rheological properties of the thermally gelling methyl cellulose made handling on the Colton machines designed for gelatin extremely difficult.
To overcome some of these problems, Greminger and Davis, U.S. Pat. No. 3,493,407, proposed the use of non-thermal gelling dip coating solutions of certain hydroxyalkylmethyl cellulose ethers in aqueous solvents. Langman, U.S. Pat. No. 3,617,588, describes the use of an induction heater to thermally gel cellulose ether dip coated pins after removal from the coating bath. However, these advances have not yet met the rigid requirements of commercial production.